Looking for some advice , Despite being my cassette being on tight to the freehub I can still move it side to side a couple of mm. The wheel was a fairly cheap Alex DM17 with I think a shimano freehub. Ive ordered a new cassette and chain but was wondering am I just as well to buy a new rear wheel than get a new freehub fitted . ?
Or am I barking up the wrong tree with thinking its the freehub that Is making the cassette wobble from side to side. By the way the freehub never slips but If I turn the bike upside down and spin the cranks backwards the chain will sag .

Many thanks .

are you missing a spacer? some go behind the cassette between the hub and cassette.

It could easily be the freehub being worn, a reasonably easy fix for about £20ish. All you need in the tool line are the usual cone spanners and a 10mm allen key to ondo the freehub. It is worth taking the freehub off first and taking it to bike shop as although they all look alike, Shimano freehubs are often subtly different.

2mm play (at teh big cog) is not unusual in freehub bearings.

By the way the freehub never slips but If I turn the bike upside down and spin the cranks backwards the chain will sag .

What happens if you spin them backwards when it’s the right way up?